Exporting OBJ mesh in blender
This issue appears when you're trying to export an object that contains no geometry, because of a single erroneous line in export_obj.py. This bug has already been reported and fixed in D5479 by Philipp Oeser. The fixed version of the exporter will be available in Blender 2.81.

In your case the object 'sphere' contains no geometry, which triggers this error. As a workaround until 2.81 is released you can either just select objects that contain geometry and enable Selection Only in the exporter or you could simply delete the problematic object. Alternatively you could also change the one line of code in the obj exporter yourself.